Method of Direct Embedding a Lithium Ion Battery on a Flexible Printed Circuit Board

ABSTRACT

A flexible printed circuit board with a lithium ion battery printed thereon is achieved. The flexible printed circuit board comprises a top and a bottom electrically insulating base film, a top electrically conductive metal layer over the top electrically insulating base film, and a bottom electrically conductive metal layer under the bottom electrically insulating base film. A printable lithium ion battery sits in a cavity completely through the top and bottom base films wherein a top of the battery contacts the top electrically conductive metal layer and wherein a bottom of the battery contacts the bottom electrically conductive metal layer. An adhesive film around the battery seals it to the top and bottom electrically insulating base film and seals the top electrically conductive metal layer to the bottom electrically conductive metal layer.

This application is a divisional of Ser. No. 16/801,779, filed on Feb.26, 2020, owned by a common assignee and herein incorporated byreference in its entirety.

TECHNICAL FIELD

This application relates to integrated circuit boards, and moreparticularly, to embedding a lithium ion battery on flexible printedcircuit boards.

BACKGROUND

With the emergence of Internet of Things (IoTs) and 5G networkingtechnologies, more and more devices are being interconnected tocommunicate with each other to make decisions and improve people'slives. Future generations of devices are expected to possess attributessuch as low cost, small form factor, high reliability,flexible/conformable, and low power consumption. Flexible electronicshave become ubiquitous in fulfilling the aforementioned challenges. Inparticular, System in Package (SiP) architecture is the packagingtechnology of choice for high level of device integration such asantennas, microprocessors, and sensors, as well as batteries to enableself-powered devices.

A battery is an essential component to power portable electronicdevices. Conventionally electronic devices have been using commercialbatteries such as prismatic, cylindrical, and coin cells. Thesebatteries are not suitable to power flexible electronics due to theirbulkiness, rigidity, and safety concerns. Power sources for flexibleelectronic devices should also conform to the devices' requirements suchas ultra-thin, ultra-light, mechanical conformity, and safety undermechanical loading.

Integrating a flexible battery directly onto a flexible substrate is anattractive solution to future generations of devices. It offers manyadvantages such as reduced form factor, reduced cost, and processsimplification. The integrated flexible battery can be used forapplications requiring power management and RF (radio frequency)communication such as a smart card, wearable devices, and internet ofthings (IoTs).

Various U.S. patent applications disclose flexible or printablebatteries including U.S. Patent Applications 2019/0273280 (Pun et al),2019/0252690 (Miles et al), 2019/0334168 (Durstock et al), 2019/0341613(Lee et al) and 2019/0355239 (Brinkley et al).

SUMMARY

A principal object of the present disclosure is to provide a method ofembedding a lithium ion battery on a flexible printed circuit board.

Another object of the disclosure is to provide a flexible printedcircuit board with a lithium ion battery printed thereon.

A further object of the disclosure is to provide a self-powered flexiblecircuit board package.

According to the objects of the disclosure, a flexible printed circuitboard with a lithium ion battery printed thereon is achieved. Theflexible printed circuit board comprises a top and a bottom electricallyinsulating base film, a top electrically conductive metal layer over thetop electrically insulating base film, and a bottom electricallyconductive metal layer under the bottom electrically insulating basefilm. A printable lithium ion battery sits in a cavity completelythrough the top and bottom base films wherein a top of the batterycontacts the top electrically conductive metal layer and wherein abottom of the battery contacts the bottom electrically conductive metallayer. An adhesive film around the battery seals it to the top andbottom electrically insulating base film and seals the top electricallyconductive metal layer to the bottom electrically conductive metallayer.

Also according to the objects of the disclosure, a method of fabricatingan electrochemical lithium ion battery in between top and bottom layersof encapsulation is achieved. An encapsulation is provided on a flexibleprinted circuit board, the encapsulation comprising top and bottomelectrically insulating base films, a top electrically conductive metallayer over the top electrically insulating base film, and a bottomelectrically conductive metal layer under the bottom electricallyinsulating base film. An anode is fabricated on the bottom metal layerof the encapsulation and the anode is electrically connected directlythrough an electrical conductive metal trace to at least one integratedcircuit chip mounted on the flexible printed circuit board as a negativeterminal allowing electrons to flow out of the anode. Alithium-metal-oxide cathode is fabricated on an aluminum layer, thealuminum layer electrically connected to the top metal layer of theencapsulation. The cathode is electrically connected directly through anelectrical conductive metal trace to the at least one integrated circuitchip mounted on the flexible printed circuit board as a positiveterminal allowing electrons to flow into the cathode. A UV-curablecomposite solid electrolyte is fabricated between the anode and thecathode. An adhesive film around the battery seals it to the top andbottom electrically insulating base film and seals the top electricallyconductive metal layer to the bottom electrically conductive metallayer.

Also in accordance with the objects of the disclosure, a method offabricating a self-powered flexible circuit board package is achieved. Aflexible printed circuit board with a printed lithium ion battery isprovided comprising a top and a bottom electrically insulating basefilm, a top electrically conductive metal layer over the topelectrically insulating base film, and a bottom electrically conductivemetal layer under the bottom electrically insulating base film. Aprintable lithium ion battery sits in a cavity completely through thetop and bottom base films wherein a top of the battery contacts the topelectrically conductive metal layer and wherein a bottom of the batterycontacts the bottom electrically conductive metal layer. An adhesivefilm around the battery seals it to the top and bottom electricallyinsulating base film and seals the top electrically conductive metallayer to the bottom electrically conductive metal layer. A plurality ofactive and passive electronic devices are mounted on top of coppertraces wherein at least one of the active devices is connected to andpowered by the printed lithium ion battery. The active devices can beintegrated circuits (IC) with different functionalities such as RF(Radio Frequency) IC, memory chips, logic IC, converter IC, powermanagement IC, application specific IC (ASIC), microcontroller unit(MCU), display driver IC, touch driver IC, touch and display driveintegration (TDDI) IC, biometrics sensor & controller IC, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of thisdescription, there is shown:

FIG. 1 is a cross-sectional representation of a preferred embodiment ofthe present disclosure.

FIG. 2 is a graphical representation of the discharging curve and thecorresponding nominal voltage of a preferred embodiment of the presentdisclosure.

FIG. 3 is a graphical representation of a cycling test of a preferredembodiment of the present disclosure.

FIG. 4 is a graphical representation of a high temperature test of apreferred embodiment of the present disclosure.

FIG. 5 is a graphical representation of a self-discharge test of apreferred embodiment of the present disclosure.

FIG. 6 is a cross-sectional representation of a completed preferredembodiment of the present disclosure.

FIG. 7 is another cross-sectional representation of a completedpreferred embodiment of the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure describes flexible printed circuit boards withflexible lithium ion batteries directly printed onto them. Flexibleelectronics have a small form factor. They typically have a high routingdensity and are foldable and bendable.

Referring now more particularly to FIG. 1, there is shown a portion of across-section of a flexible printed circuit board with printed lithiumion battery therein.

Stacked layers 10 a and 10 b on top and bottom, respectively, of theflexible printed circuit board (PCB) are electrically insulating basefilms. The base film can be polyimide (PI), liquid crystal polymer(LCP), Cyclo olefin polymer (COP), polyester (PET),polyethylene-naphtalate (PEN), poly tetra fluoro ethylene, or a laminatesubstrate such as epoxies and BT, or Teflon, or modified Teflon with athickness of between about 10 and 50 μm.

Electrically conductive metal layers 12 a and 12 b are stacked over thetop base layer 10 a and under the bottom base layer 10 b, respectively.The metal layers can be copper having a thickness of between about 5 and50 μm. A surface finishing layer is included on the metal layer toprovide printability and resistance against oxidation. The surfacefinishing layer may be nickel, palladium, gold, tin, silver, aluminum,and ruthenium or a combination of thereof.

Cavities are cut into the stacked layers 10 a and 10 b, for example, bylaser drilling or hard tool punching, until the metal layers 12 a and 12b, respectively, are exposed. An adhesive film 14 is cut, for example bylaser cutting or hard tool punching, to form an opening all the waythrough the center of the film. The cut adhesive film 14 is thenlaminated onto the bottom side metal layer 12 b within the cavity,exposing the bottom metal layer 12 b within the center opening. Batterylayers are printed onto the substrate within the cavities.

The flexible substrate is folded and laminated together with the batteryenclosed between the two sides of the folded substrate 10 a and 10 b.

The battery 20 is encapsulated by electrically insulating base film 10 aand 10 b on the outside horizontal edges of the battery and byconductive metal layers 12 a and 12 b with surface finishing layers onthe top and bottom of the battery. This encapsulation provides highresistance from water and oxygen. The combination of the electricallyinsulating base film and the metal layer have a water vapor absorptionrate no higher than 1×10⁻³ g m⁻² per day.

The adhesive film 14 between the top and bottom conductive layers 12 aand 12 b seals the layers together and provides insulation for thebattery cathode, anode, and electrolyte. The adhesive film comprisesinsulating polymer composite adhesives containing acrylic, castpolypropylene, epoxy, polyurethane or a combination of the abovesurrounding the perimeter of the battery. The adhesive film should havea dielectric constant less than 3 at a frequency of 10 GHz. If athermosetting adhesive is used, it should have a curing temperature inthe range of 150 to 200° C. and should have a peeling strength of noless than 1 N/mm with Cu metal, for example, and the electricallyinsulating base film.

The top and bottom metal layers 12 a and 12 b work as current collectorsand positive/negative terminals of the battery 20 by connecting withelectrical conductive metal traces of the positive and negativeterminals, respectively, of the flexible printed circuit board. Noexternal connection or electrical contact is needed to connect theflexible printed circuit board and the battery.

The battery 20 between the top and bottom metal layers 12 a and 12 b ofthe encapsulation includes an anode 22 fabricated on the bottom metallayer 12 b of the encapsulation. Protective layer 36 is an electricallyinsulative layer (e.g. solder resist, coverlay) that is commonly used inprinted circuit board technology to prevent electrical shorts and toprovide mechanical protection on the current collector and copper trace12 a, 12 b.

The anode 22 is electrically connected directly through an electricalconductive metal trace to the integrated circuit chips mounted on theflexible printed circuit board as a negative terminal 34 allowingelectrons to flow out of the anode. For example, an integrated circuitchip 52 is illustrated in FIG. 6 connected by metal trace 12 b to theanode 22.

The anode 22 comprises an artificial graphite in an amount of 85-90% byweight, a carbon conductive agent of Super P and KS6 in an amount of1-8% and 1-6%, respectively, and a polyvinylidene fluoride polymerbinder in an amount of 1-2%. Other anode active materials such assilicon carbon composite, graphene oxide, natural graphite, or mixturesthereof may also be used. Table 1 provides an example of anode slurryformulation.

TABLE 1 Solvent N-Methyl-2-pyrrolidone Component Weight percentage (%)Graphite 85.00%  Super P 7.89% Conductive Graphite KS6 5.53%Polyvinylidene fluoride (MW 600000) 1.58% Total  100%

A lithium metal oxide cathode 26 is fabricated on an aluminum layer 24.The aluminum layer 24 is electrically connected to the metal layer 12 aof the encapsulation through conductive adhesive 28. The metal layer 12a is electrically connected directly through an electrical conductivemetal trace to the integrated circuit chips mounted on the flexibleprinted circuit board as a positive terminal 32 allowing electrons toflow into the cathode. An example of the connection of the terminal 32with the die 52 is illustrated in FIG. 7.

The lithium-metal-oxide cathode 26 comprises a lithium metal oxide suchas LiNi_(x)Co_(y)Mn_(z)O₂, LiNi_(x)Co_(y)Al_(z)O₂, LiCoO₂,xLi₂MnO₃.(1−x)LiMO₂ (M=Mn, Ni, Co), LiMPO₄ (M=Fe and/or Mn), or LiMn₂O₄in an amount of 80-98%, a carbon conductive agent of Super P and KS6 inan amount of 1-5% and 1-5%, respectively, and a polyvinylidene fluoridepolymer binder in an amount of 1-10%. Table 2 provides an example ofcathode slurry formulation.

TABLE 2 Solvent N-Methyl-2-pyrrolidone Component Weight percentage (%)NCM 811 89.00%  Super P 3.00% Conductive graphite KS6 2.00%Polyvinylidene fluoride (MW 600000) 6.00% Total  100%

A UV-curable composite solid electrolyte 30 is fabricated on either sideof the anode and cathode electrodes 22 and 26 and is cured by UV lightwith a wavelength in the range of 200-400 nm within 1 minute. TheUV-curable composite solid electrolyte 30 has a room temperature ionicconductivity of no less than 1×10⁻⁴ S/cm after curing. As opposed to aliquid state electrolyte, the solid electrolyte of the presentdisclosure improves the battery safety significantly as no leakage/flowoccurs in the solid state.

The UV-curable composite solid electrolyte comprises a lithiumion-conducting LiPF₆ based liquid electrolyte in an amount from 75-85%by weight, a lithium ion conducting Polyethylene oxide polymer in anamount from 2-5% by weight, a Poly(vinylidenefluoride-co-hexafluoropropylene) in an amount from 1-5% by weight, anAluminium oxide ceramic filler in an amount from 2-5% by weight, anAcetonitrile solvent in an amount from 5-10% by weight, aTrimethyopropane ethoxylate monomer as polymer host in an amount from2-5% by weight, and a 2-Hydroxy-2-methylpropiophenone photo initiator inan amount from 0.01-1% by weight. Table 3 provides an example of acomposite electrolyte formulation.

TABLE 3 Component Weight percentage (%) LiPF₆ liquid electrolyte 75-85% Polyethylene oxide 2-5% Poly(vinylidene fluoride-co- 1-5%hexafluoropropylene) Aluminum oxide 2-5% Acetonitrile 5-10% Trimethyopropane ethoxylate 2-5% 2-Hydroxy-2-methylpropiophenone0.01-1%   Total 100% 

The electrochemical battery of the present disclosure has an aerialcapacity density of no less than 1.5 mAh/cm² and is capable of beingcycled at a rate of no less than 0.2 C. C-rate is a term commonly usedin the field of batteries to denote the discharge relative to itsmaximum capacity; in this case, 0.2 C means the battery is operating at20% capacity.

FIG. 2 is the discharging curve at an operation voltage in the range of3V-4V, which indicates the nominal voltage of the battery is 3.7V basedon the calculated mean value of the voltage.

FIG. 3 illustrates the capacity of the battery of the present disclosureas a function of number of cycles run (31) and Columbic efficiency (%)as a function of number of cycles (33). This graph shows that thebattery of the present disclosure has a cycle life of more than 100cycles at 0.2 C charge (20% capacity) and discharge.

A high temperature test was performed as shown in Table 4 and in FIG. 4.

TABLE 4 Test Temperature Capacity Impedance 1 Room temperature beforehigh 2.39 mAh 5.3Ω temperature 2 60° C. 7.01 mAh (293.3%) 4.41Ω 3 Roomtemperature after high 2.21 mAh 5.46Ω temperature

FIG. 4 illustrates voltage as a function of capacity for the threetemperatures in Table 4. This figure shows that the battery of thepresent disclosure can be operated at a high temperature of 60° C. withan increased capacity relative to room temperature capacity. Preferably,the battery of the present disclosure can be operated at a temperatureof about 20° C. (room temperature) up to 70° C.

A self-discharge test was performed as shown in Table 5 and FIG. 5.K-value is the average voltage drop rate in the self-discharge test;that is, the difference between initial and last open circuit voltage(OCV) divided by total time.

TABLE 5 Initial OCV % OCV Last OCV V₀ − V_(last) Hour K-Value change3.653554 V 3.643167 V 28.336 mV 846 0.03346 −0.77% mV/h

FIG. 5 illustrates open circuit voltage (OCV) as a function of time inhours at an operation voltage range of 3V-4V, which indicates thenominal voltage of the battery is 3.7V based on the calculated meanvalue of the voltage. These results show that the battery of the presentdisclosure has a low self-discharge rate of no more than 2% drop of theopen circuit voltage per month. Preferably the battery of the presentdisclosure has a working range of between about 2.5 V to 4.2 V.

FIG. 6 provides an example of a self-powered flexible circuit boardpackage including a flexible printed circuit board 10 with a printedlithium ion battery 20. First terminal 32 is connected to top currentcollector 12 a and cathode 26. Second terminal 34 is connected to bottomcurrent collector 12 b connected to anode 22. Electronic devices (activeand passive) are mounted on top of copper traces 12 a and/or 12 b. Asshown, for example, in FIG. 6, integrated circuit die 52 is mounted onthe PCB by gold bumps 42 on copper traces 40 and connected by via 38 andmetal trace 12 b to second terminal 34. Component 54, for example, isillustrated mounted by solder 44 onto copper traces 40.

FIG. 7 is another view of the flexible printed circuit board 10 withprinted lithium ion battery 20 showing integrated circuit die 52connected by via 39 and metal trace 12 b to first terminal 32.

Although the preferred embodiment of the present disclosure has beenillustrated, and that form has been described in detail, it will bereadily understood by those skilled in the art that variousmodifications may be made therein without departing from the spirit ofthe disclosure or from the scope of the appended claims.

What is claimed is:
 1. A method of fabricating an electrochemicallithium ion battery in between top and bottom layers of encapsulationcomprising: providing an encapsulation on a flexible printed circuitboard, said encapsulation comprising top and bottom electricallyinsulating base films, a top electrically conductive metal layer oversaid top electrically insulating base film and a bottom electricallyconductive metal layer under said bottom electrically insulating basefilm; fabricating an anode on said bottom metal layer of saidencapsulation and electrically connecting said anode directly through anelectrical conductive metal trace to at least one integrated circuitchip mounted on said flexible printed circuit board as a negativeterminal allowing electrons to flow out of said anode; fabricating alithium-metal-oxide cathode on an aluminum layer, said aluminum layerielectrically connected to said top metal layer of said encapsulationand electrically connecting said cathode directly through an electricalconductive metal trace to said at least one integrated circuit chipmounted on said flexible printed circuit board as a positive terminalallowing electrons to flow into said cathode; fabricating a UV-curablecomposite solid electrolyte between said anode and said cathode; andsealing said encapsulation with an adhesive film surrounding said anode,said cathode, and said solid electrolyte and vertically between said topand bottom metal layers.
 2. The method according to claim 1 furthercomprising curing said UV-curable composite solid electrolyte by UVlight with a wavelength of between about 200 and 400 nm within about 1minute.
 3. The method according to claim 1 wherein said anode comprisesan artificial graphite in an amount of 85-90% by weight, a carbonconductive agent of Super P and KS6 in an amount of 1-8% and 1-6%,respectively, and a polyvinylidene fluoride polymer binder in an amountof 1-2%.
 4. The method according to claim 1 wherein said anode comprisessilicon carbon composite, graphene oxide, natural graphite or mixturesthereof.
 5. The method according to claim 1 wherein saidlithium-metal-oxide cathode comprises a lithium metal oxide comprisingLiNi_(x)Co_(y)Mn_(z)O₂, LiNi_(x)Co_(y)Al_(z)O₂, LiCoO₂,xLi₂MnO₃.(1−x)LiMO₂ (M=Mn, Ni, Co), LiMPO₄(M=Fe and/or Mn), or LiMn₂O₄in an amount of 80-98%, a carbon conductive agent of Super P and KS6 inan amount of 1-5% and 1-5%, respectively, and a polyvinylidene fluoridepolymer binder in an amount of 1-10%.
 6. The method according to claim 1wherein said UV-curable composite solid electrolyte has a roomtemperature ionic conductivity no less than 1×10⁻⁴ S/cm after saidcuring.
 7. The method according to claim 1 wherein said UV-curablecomposite solid electrolyte comprises a lithium ion-conducting LiPF6based liquid electrolyte in an amount from 75-85% by weight, a lithiumion conducting Polyethylene oxide polymer in an amount from 2-5% byweight, a Poly(vinylidene fluoride-co-hexafluoropropylene) in an amountfrom 1-5% by weight, an Aluminium oxide ceramic filler in an amount from2-5% by weight, an Acetonitrile solvent in an amount from 5-10% byweight, a Trimethyopropane ethoxylate monomer as polymer host in anamount from 2-5% by weight, and a 2-Hydroxy-2-methylpropiophenone photoinitiator in an amount from 0.01-1% by weight.
 8. The method accordingto claim 1 wherein said electrochemical lithium ion battery has anaerial capacity density no less than 1.5 mAh/cm² and is capable of beingcycled at a rate of no less than 0.2 C.
 9. The method according to claim1 wherein said electrochemical lithium ion battery can be operated at anominal voltage of 2.5 V to 4.2 V.
 10. The method according to claim 1wherein said electrochemical lithium ion battery has a cycle life OFmore than 100 cycles at 0.2 C charge and discharge.
 11. The methodaccording to claim 1 wherein said electrochemical lithium ion batterycan be operated at a high temperature of up to 70° C. with an increasedcapacity relative to room temperature capacity.
 12. The method accordingto claim 1 wherein said electrochemical lithium ion battery has a lowself-discharge rate of no more than a 2% drop of the open circuitvoltage per month.
 13. A method of fabricating a self-powered flexiblecircuit board package comprising: providing a flexible printed circuitboard with a printed lithium ion battery according to claim 1; andmounting a plurality of active and passive electronic devices on top ofcopper traces wherein at least one of said active devices is connectedto and powered by said printed lithium ion battery.
 14. The methodaccording to claim 13 wherein the said active electronic devices areintegrated circuits (IC) with different functionalities including one ormore of: RF (Radio Frequency) IC, memory chips, logic IC, converter IC,power management IC, application specific IC (ASIC), microcontrollerunit (MCU), display driver IC, touch driver IC, touch and display driveintegration (TDDI) IC, and biometrics sensor and controller IC.